The present invention relates to scintillator compositions and related devices and methods. More specifically, the present invention relates to scintillator compositions including a scintillation compound and a dopant for use, for example, in radiation detection, including gamma-ray spectroscopy, and X-ray and neutron detection, imaging applications, and the like.
Scintillation spectrometers are widely used in detection and spectroscopy of energetic photons (e.g., X-rays, gamma-rays, etc.). Such detectors are commonly used, for example, in nuclear and particle physics research, medical imaging, diffraction, non destructive testing, nuclear treaty verification and safeguards, nuclear non-proliferation monitoring, and geological exploration.
Important requirements for the scintillation crystals used in these applications include high light output, transparency to the light it produces, high stopping efficiency, fast response, good proportionality, low cost, and availability in large volume. These requirements on the whole cannot be met by many of the commercially available scintillator compositions. While general classes of chemical compositions may be identified as potentially having some attractive scintillation characteristic(s), specific compositions/formulations having both scintillation characteristics and physical properties necessary for actual use in scintillation spectrometers and various practical applications have proven difficult to predict. Specific scintillation properties are not necessarily predictable from chemical composition alone, and preparing effective scintillator compositions from even candidate materials often proves difficult. For example, while the composition of sodium chloride had been known for many years, the invention by Hofstadter of a high light-yield and conversion efficiency scintillator from sodium iodide doped with thallium launched the era of modern radiation spectrometry. More than half a century later, thallium doped sodium iodide, in fact, still remains one of the most widely used scintillator materials. Since the invention of NaI(Tl) scintillators in the 1940's, for half a century radiation detection applications have depended to a significant extent on this material. The fields of nuclear medicine, radiation monitoring, and spectroscopy have grown up supported by NaI(Tl).
Although far from ideal, NaI(Tl) was relatively easy to produce for a reasonable cost and in large volume. With the advent of X-ray CT in the 1970's, a major commercial field emerged as did a need for different scintillator compositions, as NaI(Tl) was not able to meet the requirements of CT imaging. Later, the commercialization of positron emission tomography (PET) imaging provided the impetus for the development of yet another class of detector materials with properties suitable for PET. As the methodology of scintillator development evolved, new materials have been added, and yet, specific applications are still hampered by the lack of scintillators suitable for particular applications.
As a result, there is continued interest in the search for new scintillator compositions and formulations with both the enhanced performance and the physical characteristics needed for use in various applications. Today, the development of new scintillator compositions continues to be as much an art as a science, since the composition of a given material does not necessarily determine its properties as a scintillator, which are strongly influenced by the history (e.g., fabrication process) of the material as it is formed. While it may be possible to reject a potential scintillator for a specific application based solely on composition, it is typically difficult to predict whether even a material with a promising composition can be used to produce a useful scintillator with the desired properties.
One of the uses of radiation monitoring devices is preventing the spread of weapons of mass destruction such as nuclear weapons. One way to passively determine the presence of nuclear weapons is to detect and identify the characteristic signatures of special nuclear materials (SNMs) such as highly enriched uranium and weapons grade plutonium. Characteristic X-rays and gamma-rays are signatures of these materials. The general approach to a passive gamma-ray assay is to acquire raw spectra, correct the spectra for rate-related electronic losses and source attenuation, and compute the total corrected count rate, which is proportional to the mass of the isotope being assayed. The proportionality constant includes the effects of gamma-ray emission rate, solid angle, and detector efficiency.
Monitoring for both highly enriched uranium and weapons grade plutonium involves analysis of X-ray and gamma-ray spectra with multiple energies of interest. One important consideration in SNM monitoring is the determination of uranium enrichment since highly enriched uranium can be used for development of nuclear weapons. The naturally occurring isotopic abundance of uranium is: 238U (99.27%), 235U (0.720%) and 234U (0.006%). When the fraction of the fissile 235U is higher than that in naturally occurring uranium, the uranium is said to be enriched. The relative intensity of 185.7 keV gamma-rays (from 235U) compared to the 94-98 keV X-rays for uranium in an unattenuated spectrum can be used to determine uranium enrichment (Passive Nondestructive Assay of Nuclear Materials, eds. Reilly et al., U.S. Nuclear Regulatory Commission, Washington D.C., pp. 11-18, (1991)). As the uranium enrichment level increases, the relative intensity of the 185.7 keV gamma-ray peak increases in comparison to the 94-98 keV X-ray peak. Another consideration in SNM monitoring is to distinguish “weapons-grade” plutonium (with ≧93% 239Pu) from “reactor-grade” plutonium <60% 239Pu). The “reactor-grade” plutonium includes other isotopes such as 240Pu, 241Pu, 242Pu, and 238Pu. Comparison of gamma-ray signatures of 239Pu (such as 129.3 keV and 413.7 keV photons) with those for other plutonium isotopes allows determination of the grade of plutonium. In addition to the characteristic X- and gamma-rays of highly enriched uranium and weapons grade plutonium, there is considerable interest in the measurement of irradiated fuel from nuclear reactors because of the plutonium produced during reactor operation. Due to very intense gamma-rays emitted by fission products of the irradiated fuel, gamma-rays from spontaneous decay of plutonium and uranium (235U, 239Pu and 241Pu) are generally not used for measurement of irradiated fuel. The most commonly measured fission product gamma-ray is the 662 keV line from 137Cs. Recent threat of “dirty bombs” (devices which spread radioactive material using conventional, non-nuclear explosives) has also created an interest in monitoring of radioactive materials such as 137Cs, 60Co, 241Am, radioactive medical waste, and irradiated fuel from nuclear reactors that emit high energy gamma-rays.
Thus, gamma-ray spectrometers and radiation detectors are important tools in monitoring of special nuclear materials. A number of homeland security systems such as hand-held radioisotope identifiers, vehicle portals for radiation detection, and personal radiation detection devices rely on availability of high performance gamma-ray spectrometers. Similar systems are also required for nuclear non-proliferation monitoring. An important challenge in homeland security monitoring is not only to detect hidden radioactive materials but also to distinguish them from routinely used radiopharmaceuticals as well as from naturally occurring benign radioactive materials.
Existing scintillator materials and commercial radiation detectors do not meet the current needs for radiation monitoring and weapons detection. For example, existing scintillator compositions and detectors typically lack the one or more important scintillation properties (e.g., high energy resolution, light output, stopping power, fast response, and the like) that are desired and/or are not useful in detecting both energetic photons (e.g., gamma-rays and X-rays) as well as neutron emission. Thus, a need exists for improved scintillator compositions suitable for use in various radiation detection applications, including, for example, radiation and nuclear weapons monitoring.